This invention is directed towards a resistor embedded in a printed circuit board and more particularly to a method and apparatus for forming a termination resistor in a printed circuit board.
A number of integrated circuits require connection to a termination voltage on a printed circuit board. As shown in FIG. 1, an output driver 12 from an integrated circuit provides signals on a transmission line 10. The signals are received by an input driver 14 and provided to additional circuits. It is required in many applications to connect the transmission line 10 to a termination resistor 16 shown as RT. The other end of the termination resistor is connected to a termination voltage, VT, 18. In many applications, the termination voltage VT is ground. In other applications, the termination voltage may be a voltage other than ground, such as xe2x88x925 volts, +2 volts or some other acceptable value.
FIG. 2 illustrates one prior art technique for providing this termination resistor. According to this technique, an integrated circuit is mounted on a printed circuit board 20. The printed circuit board 20 includes a transmission line 10 providing an input connection to the integrated circuit 22. A resistor has a first end connected to the transmission line 10 coupled to the circuit 22 and the second end connected to the termination voltage. In many applications, this is done by mounting a separate resistor on the printed circuit board and electrically connecting the resistor to a pin on the chip and mounting the chip on the printed circuit board. An array of resistors providing the desired termination resistor value are mounted on the printed circuit board and made available for such connections. This is a conventional approach and is used in many applications today. A disadvantage of this approach is the cost and effort of connecting the required individual pins from the integrated circuit board 22 to the termination resistor. In addition, additional room must be found on the printed circuit board 20 for mounting the resistors. Since resistors are exposed, they are subject to mechanical and physical damage. In the event a resistor becomes defective or a connection to the resistor is broken, often the whole board is usually discarded.
FIG. 3 illustrates a second method of providing the termination resistor according to the prior art. According to this technique, a resistor 16 is formed internal to the integrated circuit. The input line 10 is connected to the integrated circuit and directly connected to a first end of the resistor 16. The line 10 is also provided to other parts of the integrated circuit as electrically shown in FIG. 1. In addition, the other end of the resistor 16 is connected to a termination voltage via an additional pin output from the integrated circuit. The structure used in the prior art of FIG. 3 has the advantage of being more compact and more easy to manufacture because the resistor is formed as part of the integrated circuit manufacturing process. On the other hand, it has the disadvantage of requiring additional pins out to the termination voltage potential. It may cause parasitic problems on the integrated circuit along with consuming precious silicon area on the integrated circuit. In addition, forming precise resistors having large current carrying capability on an integrated circuit is difficult. Accordingly, while this technique has some advantages, it also has a number of other disadvantages which prevent it from being used in many applications.
According to principles of the present invention, termination resistors are formed in one or more layers in a printed circuit board. A conductive layer in the printed circuit board is composed of two metals, a highly conductive metal such as copper, and a less conductive metal such as nickel chromium, or other desired material to act as a resistor. The conductive layer composed of two metals is formed on top of an insulating layer.
After the conductive layer is formed on the printed circuit board, a portion of the highly conductive material is removed at a selected location so as to leave only a resistive layer of metal. A resistor is thus provided forming a series electrical path from a first portion of the highly conductive layer to a second portion of the highly conductive layer. An insulating layer is then overlaid on top of the conductive layer. Additional conductive and insulating layers are formed as part of the printed circuit board. After the printed circuit board is formed, an aperture is made through the printed circuit board. The inside of the aperture is coated with metal to connect the conductive layer to a second conductive layer in the printed circuit board. The second conductive layer is connected to a pin on an integrated circuit. One side of the conductive layer is connected to the termination voltage and the other side of the conductive layer is electrically connected to the pin of the integrated circuit for which a termination voltage is required.
The location of the resistive portion of the conductive layer that forms a resistor is positioned sufficiently far from the electrical connector to avoid mechanical stress on the resistor when the aperture is formed and also to provide thermal isolation between the resistor and the electrical connector.
The present invention has the advantage of providing at very low cost the potential for many thousands of resistors within the printed circuit board at selected locations. Further, the resistor is very easily formed and its value can be determined with precision by simple metal etching techniques. All resistors in the printed circuit board are etched in the very same step, thus providing precision of the resistor values.